Siddikur put himself in prime position to win a second Asian Tour title after firing five birdies against a lone bogey at the Delhi Golf Club to take the lead from Lahiri, who birdied his last two holes for a bogey-free 66.
Joint overnight leader Khalin Joshi battled to a 71 to end the day two strokes back in the USD 300,000 Asian Tour event, which is celebrating its sixth edition.
Thailand's Chapchai Nirat and home hero Shiv Kapur will enter the final day four off the lead after rounds of 67 and 69, respectively, while American Chan Kim stumbled to a 74 to fall out of contention.
The 29-year-old Siddikur, the first Bangladeshi to win on the Asian Tour when he won the Brunei Open in 2010, was delighted to push ahead of the home favourites.
"Very consistent. I'm so happy," smiled Siddikur, who hit 12 fairways and 14 greens in regulation. "I've been very consistent. I'm happy with myself with three 68s. I've been trying to get more regulation play and it's been working."